The following invention relates to a connector for bonding IC dies directly to a substrate and more particularly to a connector and the method of making it which permits the direct bonding of IC dies to a substrate without the need for wire bonds.
IC dies are typically mounted on a substrate using one of two methods. When the substrate is glass, typically an IC die is placed upon a plastic mounting on the glass and a wire bond is used to connect the bond pads of the IC die to metal leads that extend across the glass substrate to the edge of the plastic mounting. The metal leads are connected by way of elastomers to the edge interconnects that lead away from the substrate. More recently, bonds called "FlipChip" interconnects have been used in which metal circuit runs which extend across a glass substrate and are connected to IC dies using a gold bump at the end of the circuit runs which mates with bond pads on the IC die. The IC die is compressed onto the gold bumps and an adhesive is then applied around the bumps. The problem with this method is that the adhesive must have an extremely low thermal expansion coefficient.
Solid metal bumps have a very limited elastic range. The preferred metals, gold, indium, or copper, have fairly low coefficients of thermal expansion. For example, the C.sub.TE of gold is 14.2.times.10.sup.-6. On the other hand, adhesives typically have C.sub.TE 's in the range of 114.times.10.sup.-6 to 550.times.10.sup.6. Thus, the adhesive expands much faster than the gold bumps as temperature rises. Gold has a Youngs Modulous of only 11.6.times.10.sup.-6 psi. This means that the bump might be unable to elastically expand and will lift off of the substrate as the temperature is increased. Water absorption is also a problem because water can be absorbed by the adhesive causing it to swell, and the swell in turn lifts the bump off of the substrate and its interconnecting circuit run. Thus, a great deal of time and effort has been invested in the past in attempting to design an adhesive that has a low enough coefficient of thermal expansion to be used with a FlipChip mounting.